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Install a supported Python 3 interpreter, verify the command your terminal uses, then create a project-local environment with python -m venv .venv. Activate that environment before installing packages, and use python -m pip so each dependency is written to the project environment rather than to a system-wide Python installation.
This guide covers the standard-library workflow on Windows, macOS and Linux. It explains how to select the right interpreter, handle Windows PowerShell execution policy, record dependencies, rebuild a damaged environment and diagnose the most common setup errors.
What you are installing
Python is the interpreter that runs .py programs and the package-management commands used by those programs. A virtual environment is a separate directory containing an environment-specific Python configuration and package location. The Python documentation describes venv as creating lightweight environments with independent sets of packages in their own “site” directories.
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A virtual environment is not a second operating-system installation of Python. It is created from a Python interpreter that is already installed on the computer. If Python is absent, python -m venv .venv cannot work until you install Python first.
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When the environment is active, package installs normally go into that environment. Project A can therefore use one version of a library while Project B uses another, without changing the base installation or other environments. The isolation is intended for project dependencies; it is not a security boundary and does not replace normal operating-system permissions or application security.
Choose and install a Python interpreter
Use an official Python distribution or an operating-system package source appropriate to your platform. The exact installer, package name and command differ by operating system and Linux distribution, so do not copy a single Linux command as if it were universal. The Python Packaging User Guide distinguishes official downloads from OS-managed installations and recommends checking current platform instructions rather than relying on an old release number.
| Platform or route | What to do | Important qualification |
|---|---|---|
| Windows | Install a supported Python 3 release using the current official Windows instructions. If the installer offers an option to add Python to your PATH, understand that this changes which command your terminal can find. | The Python launcher command py is commonly available on Windows, but it is not guaranteed on every installation. |
| macOS | Use the current official macOS installation guidance or a package source you intentionally manage. Afterward, check whether your terminal invokes the installed interpreter as python or python3. |
Do not assume that a pre-existing system or developer-tool Python is the version you intend to use. |
| Linux | Follow the instructions for your distribution and release, then verify the resulting command. Distribution repositories, package names and Python command aliases vary. | No single apt, dnf, pacman or other command is correct for every Linux system. |
| Managed or restricted computer | Use the approved software channel or ask the administrator to install a supported interpreter. | You may not be allowed to modify PATH, install packages or change PowerShell policy. |
The Packaging User Guide currently describes its tutorial as applying to supported Python versions, listed there as Python 3.8 and higher at the time of that guidance. Supported versions change, so check the live documentation when choosing a new installation. The venv module itself has been part of Python since Python 3.3.
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Verify the interpreter before creating anything
Open a new terminal after installation so it reads any PATH changes. Try the command that matches your platform:
python --versionis common when the installer created apythoncommand.python3 --versionis common on Unix-like systems wherepythonis reserved for another installation or is not defined.py --versionis available on Windows installations that include the Python launcher.
Use the command that prints the supported Python 3 version you intend to use. If a command reports that it cannot be found, Python is either not installed, the terminal has not been reopened, or the interpreter directory is not on PATH. Recheck the platform-specific installation instructions before creating a virtual environment.
For a definitive check of the executable selected by a command, ask Python to print its path:
python -c "import sys; print(sys.executable)"Substitute python3 or py if that is the command that successfully reports your version. This check matters when several Python installations are present: the environment will be based on the interpreter you actually invoke.
Create a project-local environment
Keep the environment inside the project directory and use the conventional name .venv. The leading dot keeps it unobtrusive in directory listings, while the name makes the purpose clear to collaborators and tools.
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- Create or open the directory for your project.
- Change the terminal’s current directory to that project directory.
- Run the environment command with the interpreter you verified:
python -m venv .venvIf your working interpreter is versioned, use its command instead, for example:
python3 -m venv .venvOn a Windows installation that uses the launcher, this equivalent form selects a Python installation through py:
py -m venv .venvCreating the directory can take a moment. A successful run normally returns to the prompt without a message. The new .venv directory contains the environment’s scripts and configuration; it does not contain your project source code and it does not install third-party packages by itself.
If a directory named .venv already exists, do not blindly overwrite it. First determine whether it belongs to this project and whether it was created by the Python interpreter you want. A moved or copied environment can retain absolute paths from its original location. When in doubt, remove the old environment directory and create a new one in the project directory instead of trying to repair or relocate it.
Activate the environment for your shell
Activation adjusts the current shell so that commands such as python and pip resolve to the environment. It does not permanently alter the base installation. Use the script that matches both your operating system and your shell.
Unix-like systems using bash or a compatible shell
source .venv/bin/activateThis form is the documented example for bash on macOS and Linux. The environment name usually appears in parentheses at the start of the prompt, such as (.venv). That prompt marker is a convenience, not proof by itself; verify the interpreter in the next section.
Windows PowerShell
.venvScriptsActivate.ps1PowerShell can block local scripts under its execution-policy settings. If activation is refused and you are permitted to change the policy for your account, the venv documentation gives this user-scoped option:
Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUserCurrentUser limits the change to your Windows account rather than changing policy for every user on the computer. RemoteSigned allows locally created scripts while applying signature rules to scripts obtained from elsewhere. This is a configuration choice with security implications; do not bypass policy with a system-wide change merely to follow a beginner tutorial. On a managed computer, ask the administrator for the approved procedure.
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Windows Command Prompt
.venvScriptsactivate.batCommand Prompt and PowerShell use different activation scripts. Running a PowerShell command in Command Prompt, or vice versa, produces a confusing “not recognized” or script error even when the environment itself is valid.
Other Unix shells
The tutorial’s source example is for bash and compatible shells. The environment also includes shell-specific activation scripts for shells such as csh and fish. Use the script supplied for the shell you are actually running rather than adapting bash syntax by guesswork. You can also invoke the environment’s interpreter directly without activation when a script or automation workflow requires that approach.
Confirm that activation selected the right Python
With the environment active, run the same executable check again:
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python -m pip --versionThe reported pip location should be associated with .venv, not a system or user package directory. If it points elsewhere, activation did not take effect or you are using a different terminal session. Close and reopen the shell if necessary, activate again, and repeat both checks.
Install project packages safely
Install a package only after the environment is active and its interpreter has been verified. The packaging guides use this pattern:
python -m pip install requestsrequests is only an example; replace it with a dependency your project actually needs. Calling pip as a module of the selected Python is safer than typing a standalone pip command when multiple Python installations exist, because the module invocation ties pip to the interpreter named by python.
After installation, the package is available to programs run with that environment’s Python. A package installed while the environment is inactive may instead be placed in the base interpreter or a user-level location, leaving the project unable to import it. If an import fails, check the executable and pip paths before installing the package again.
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Record dependencies so the environment can be rebuilt
A virtual environment is disposable. Record the packages and versions your project requires in a requirements file kept with the project source. A simple file might contain one dependency per line:
requests
Use the active environment to install the recorded set into a newly created environment:
python -m pip install -r requirements.txtThe file is project information; .venv is generated state. Keep the requirements file under version control, but exclude the environment directory itself. A typical ignore entry is:
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Use, leave and recreate the environment
Run project commands
While the environment is active, run your application, tests and package commands with python. This keeps all of those operations tied to the same interpreter and package set that you checked with sys.executable.
Deactivate when finished
To return the current shell to its previous command resolution, run:
deactivateThis does not delete the environment or uninstall packages. It only removes the activation changes from the current shell. A later terminal session must be activated again before project-specific packages are available by default.
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Rebuild instead of moving or copying
Virtual environments are designed to be recreated, not moved between directories or copied to another computer. If the project path changes, the interpreter changes, or the environment becomes inconsistent, create a new .venv in the current project directory and install the requirements file. This approach also provides a clean recovery from accidental package changes.
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- Deactivate the old environment if it is active.
- Remove the old
.venvdirectory using your operating system’s normal file-management method. - Run the appropriate
python -m venv .venv,python3 -m venv .venvorpy -m venv .venvcommand from the project directory. - Activate the new environment with the script for your shell.
- Install the recorded dependencies with
python -m pip install -r requirements.txt.
Troubleshoot common setup failures
“Python” or “python3” is not recognized
The interpreter is missing, the command name differs on your platform, or the terminal has stale PATH information. Try the alternative command listed earlier, open a new terminal, and verify the installation using the platform’s current official instructions. On Windows, check whether py is available. On Linux, consult the documentation for your exact distribution instead of applying a command from another distribution.
venv cannot be found
Make sure you invoked the intended Python 3 interpreter and that the command is being run from the project directory. A very old or unusual Python installation may not provide the expected standard-library module. Install or select a supported Python version, verify its executable path, then retry the creation command.
Activation script is not recognized
Confirm the path and shell: PowerShell uses .venvScriptsActivate.ps1, Command Prompt uses .venvScriptsactivate.bat, and bash uses source .venv/bin/activate. Run the command from the directory that contains .venv. If PowerShell reports a policy restriction, consider the user-scoped RemoteSigned setting described above or follow your organization’s policy.
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Check both executable paths while the environment is active:
python -c "import sys; print(sys.executable)"
python -m pip --versionIf either path is outside .venv, activate the environment again and use python -m pip install .... Also ensure that the application is launched with the same environment’s Python rather than with a separately configured interpreter in an editor or task runner.
The environment worked until the project was moved
Do not copy or patch the old directory. Recreate .venv at the new location and reinstall from the requirements file. This avoids stale path references and restores the intended project-local layout.
venv versus virtualenv
For ordinary project isolation, the standard-library venv module is the simplest starting point. The packaging guide also documents virtualenv, a separately installed alternative. Choose based on the project’s compatibility and workflow needs rather than adding a second tool automatically.
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virtualenv |
|---|---|---|
| Installation | Included with Python 3.3 and later, so no separate environment tool is required for the normal case. | Must be installed separately before it can create environments. |
| Basic project setup | Provides the standard command, python -m venv .venv. |
Provides an alternative workflow when a project specifically calls for it. |
| Compatibility decision | Best default when the selected supported Python supplies venv. |
Worth evaluating when the project needs features or Python-version compatibility beyond the standard-library workflow. |
| What both provide | An isolated package location that keeps project dependencies separate from the base installation and other environments. | |
Installing an additional tool does not solve a missing base interpreter. Both approaches require Python to be installed first, and both still require you to select, activate and verify the environment used by your project.
A practical setup checklist
- Install a currently supported Python 3 interpreter using the instructions for your operating system.
- Open a fresh terminal and confirm the version and executable path.
- Change to the project directory before creating the environment.
- Create
.venvwith the command belonging to the intended interpreter. - Activate it with the script for the shell you are actually using.
- Verify that both
sys.executableandpython -m pip --versionpoint into.venv. - Install only the project’s required packages through
python -m pip. - Record dependencies in a requirements file and use that file to rebuild a fresh environment.
- Exclude
.venv/from version control. - Deactivate when finished, and recreate rather than move an environment when its path or interpreter changes.
Once these checks pass, Python itself is installed at the system level and your project has a separate, reproducible package area. Future projects can repeat the same pattern with their own .venv directory and dependency file.
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